Traditionally, earthworms are categorized into three main ecological groups—epigeic, endogeic, and anecic, based on their morphology, location in the soil profile, and feeding habits. Recently, more refined groups have been proposed and widely used in earthworm studies. However, even though the categorization system is simple and convenient, many earthworm species do not fit well into these pre-defined groups. So in this study, we attempted to revisit the traditional categorization system by analyzing the carbon and nitrogen stable isotopes of 10 earthworm species commonly found in the arable fields and deciduous forests in temperate North America.
Our results revealed several interesting patterns regarding the isotopic niches of earthworms in the δ13C-δ15N space. First, rather than being clustered into distinct categories, the isotopic niches of different ecological groups actually formed a continuum, ranging from species feeding on leaf litter to those feeding on highly processed soil organic matter. Second, along this continuum, species of the same ecological group can exhibit considerable isotopic niche differentiation and resource partitioning. For example, we found that two closely related endogeic species, Aporrectodea caliginosa and Aporrectodea trapezoides, showed substantial niche partitioning (low isotopic niche overlaps) in the arable field. Third, species within the same ecological group can be further classified as trophic generalists or specialists based on their niche breadths. At an abandoned arable field, there were three endogeic species—Allolobophora chlorotica occupied a larger isotopic niche area and was thus a generalist, whereas Aporrectodea caliginosa and Aporrectodea trapezoides occupied smaller isotopic niche areas and were both specialists in a relative sense.
Taken together, our results suggest that, despite their convenience, there are limitations to the traditionally-used ecological categories of earthworms. We propose to incorporate stable isotopes as functional traits (trophic traits) into studies on earthworm ecology and encourage research to further expand this isotopic approach worldwide to better understand the linkage between earthworm communities and ecosystem functions.
Currently under review. Stay tuned!!!